Biological nutrient removal (BNR) systems are used in the treatment of wastewater containing nitrogen (N) and phosphorous (P). N removal is conventionally accomplished using a two stage treatment process. In the first stage, aerobic nitrification is used to oxidize ammonia to nitrite and then to nitrate. The effluent of this process is then direct to a second stage, in which anoxic de-nitrification is used to reduce nitrate to nitrite and then to N2 gas. In continuous flow systems, the two stage process normally is conducted in two separate vessels or two separate zones within a single vessel. It is also known to conduct simultaneous nitrification-denitrification (SND) in which N removal is achieved in a single anoxic reactor through partial oxidation of ammonium to nitrite, which is then directly converted to N2 gas. However, regardless of the process configuration, the de-nitrifying bacteria require a source of carbon for cell growth and it is typically the availability of a carbon substrate in the de-nitrification stage that limits the overall rate of N removal.
P removal is achieved through enhanced biological phosphorus removal (EBPR) under alternating anaerobic-aerobic conditions using polyphosphate accumulating organisms (PAO's). Phosphate is normally released by PAO's under anaerobic or anoxic conditions and accumulated under aerobic conditions. When appropriate process parameters are selected (for example, solids retention time (SRT), hydraulic residence time (HRT), chemical oxygen demand (COD), pH, temperature, and the like), the accumulation of phosphorous exceeds the rate of release and the net effect is the accumulation of phosphorous in the aerobic bio-reactor sludge. Since biomass formation is reliant upon the availability of a carbon substrate, the capacity of the system for phosphorous uptake is dependent upon the concentration of organic carbon, expressed as COD. It is typically the availability of COD that limits the overall rate of phosphorous removal in EBPR systems.
Biological processes for the combined treatment of wastewater containing phosphorous and nitrogen are known. Examples of these include the A2O, Bardenpho, Johannesburg, University of Cape Town (UCT), and modified UCT (MUCT) processes. In these processes, phosphorous uptake predominantly occurs in an aerobic treatment step following de-nitrification of the effluent. However, since de-nitrification and phosphorous uptake both consume the same organic carbon substrate, the rate of phosphorous removal is typically limited by the availability of COD in the wastewater. This slow rate of P removal increases the required hydraulic residence time of the process.
Many of these conventional BNR processes for combined N and P removal utilize a series of bio-reactors. The desired residence time in each bio-reactor is adjusted by controlling the influent flow rate. Since phosphorus is removed only through sludge disposal, a short SRT is desirable; however, for nitrogen removal through nitrification, a longer SRT is needed due to the slow growth of the nitrifiers. Thus, there is a SRT conflict between the N and P removal operations. Control of proper oxidation-reduction potential in each bio-reactor is difficult to achieve using flow rate alone. As a result, it is desirable to use multiple bio-reactors linked with recycle streams for BNR processes.
It has been reported that denitrification can be accomplished in anaerobic-anoxic EBPR systems using denitrifying phosphorous accumulating organisms (DPAOs), which perform simultaneous nitrate/nitrite reduction and P uptake using the same organic substrate (Kuba et al., 1993; Kerrn-Jespersen et al., 1994). Use of DPAOs results in lower COD demand, reduced aeration cost, and less sludge production than in conventional BNR systems. Competition for carbon amongst metabolic pathways of these organisms typically favours de-nitrification over phosphorous accumulation in the presence of nitrate. Gerber et al. (1986, 1987) has stated that, for DPAO's, the availability of organic substrate determines whether or not a net P uptake or P release is observed under anoxic conditions in the presence of nitrate. Out of the twelve different substrates tested, P release in the presence of nitrate occurred with formate, acetate and propionate; however, all other substrates tested (glucose, methanol, settled sewage, TCA cycle intermediates (citrate and succinate) and a number of glucose fermentation products) failed to induce P release until nitrate was consumed. The rate of phosphorous uptake is related to the rate of phosphorous release and therefore if the rate of phosphorous release is retarded, the rate of uptake will also be compromised. As a result, when used in conventional processes that conduct aerobic nitrification prior to phosphate removal, DPAO's are typically incapable of simultaneously removing phosphates and nitrates to the level required for meeting practical treatment objectives.
The Dephanox process (Bortone, et al., 1996) makes use of DPAO's to conduct phosphorous uptake in an anoxic bio-reactor. In the Dephanox process, phosphorous removal is preceded by nitrification, as is conventionally done, using a fixed film aerobic bio-reactor. The fixed film reactor type is chosen to maintain the concentration of nitrifiers in the aerobic bio-reactor. The effluent of the fixed-film aerobic bio-reactor flows directly to the anoxic bio-reactor. To make up for the organic carbon consumed during de-nitrification, sludge from a clarifier preceding the aerobic bio-reactor is provided to the anoxic bio-reactor. The anoxic bio-reactor thus treats both the aerobic bio-reactor effluent and the clarifier sludge. The anoxic bio-reactor causes phosphorous uptake and is followed by a suspended growth second aerobic bio-reactor for regeneration of the DPAO's through oxidation of intracellular phosphorous storage products. A second clarifier is used after the suspended growth second aerobic bio-reactor to create an aerobic bio-reactor sludge that is returned to the head of the process for anaerobic treatment with the incoming wastewater. However, the presence of nitrates in the nitrified effluent from the second aerobic bio-reactor, which is also recycled to the anaerobic bio-reactor, limits the amount of P release in the anaerobic bioreactor. This limitation occurs because most of the readily biodegradable organic carbon is utilized for denitrification in the anaerobic bioreactor, leaving less readily biodegradable organic carbon for anaerobic P release. As a result, the overall P removal in the Dephanox process is compromised. This is true of any BNR process that recycles nitrates with return activated sludge to the anaerobic bioreactor.
In aerobic wastewater treatment, membranes are sometimes used to improve the effectiveness of conventional suspended growth systems, such as activated sludge (AS). Suspended growth aerobic rectors incorporating membranes are referred to as membrane bio-reactors (MBR's). MBR's offer many benefits over conventional suspended growth systems, such as: small footprint and HRT requirements; better effluent water quality; disinfection; less sludge production; and the ability to retain biomass within the reactor, facilitating more precise control of SRT and leading to improved operational stability and robustness. For biological N removal, the long SRT and high-suspended solids concentration achieved in the MBR prevents wash out of slow-growing nitrifying bacteria and also improves the nitrifying capability of the system. Since most MBR systems employ a single reactor, the same SRT applies for nitrification and denitrification; this necessitates a large system, since aerobic nitrification is the rate limiting step. While MBRs have been demonstrated for biological N and P removal individually (Fan et al., 1996, Cicek et al., 1999), combined biological N and P removal in MBR's has not been demonstrated.
Several attempts have been made to use MBR processes for BNR. U.S. Pat. No. 6,485,645 (Husain, et al.) discloses a biological process for removing nitrogen through nitrification-denitrification and phosphorus using enhanced biological phosphorous removal (EBPR). The aerobic removal of phosphorous by EBPR produces more sludge, than anoxic P uptake, increases the demand for organic carbon, and requires much higher aeration energy. In particular, the process employs an anaerobic reactor that represents only about 9% of the total bioreactor volumes, corresponding to an HRT<1 hour, which is too small to provide significant P treatment. A membrane filter is disclosed for use in the aerobic nitrification reactor. This process is inefficient for treating wastewater containing both phosphorous and nitrogen, particularly where biodegradable organic carbon is limited.
The need therefore still exists for an improved method and process for the treatment of wastewater containing phosphorous and nitrogen.